1. Field of the Invention
This invention relates generally to the apparatus and method of a down converter for microwave signal transmission. More particularly, this invention relates to an improved antenna structure and method to design and manufacture a single body unit of downconverter integrated with a dipole antenna. The unit is provided with improved dipole structure and a novel semi-mechanical filter for application in a television signal reflector and antenna system to improve the reliability and filtering performance of the signal process.
2. Description of the Prior Art
For television signal transmission, several technical difficulties are faced in the application of a conventional down converter for typical semi-parabolic or dish-shaped antennas. The design involves a feed antenna integrated with a down converter. The down converter, which is integrated with a dipole antenna and implemented as part of the semi-parabolic antenna as a single operation unit is commonly installed on a roof top to operate in an outdoor environment. In order to insulate the dipole antenna and the down converter from water damages, special packaging material such as certain plastic container and fillers injected into a housing structure are required. The difficulties arise from the fact that the performance characteristics of the dipole antenna are often altered significantly during the filler injection process depending on various filler injection parameters. While the functional relationship between the performance characteristics and the parameters applied in the filler injection process are difficult to measure and control. A dipole antenna has to be designed and manufactured through several trial-and-error iterations before a dipole antenna with precise performance characteristics can be achieved and ready for packaging by employing a plastic injection process. Thus, the dipole antenna implemented with the plastic injection molding package are generally considered as inconvenient and expensive due to the requirement of applying this trial-and-error iterative manufacture process. In addition to the technical difficulties faced by those involved in manufacturing the dipole antenna, a mechanical filter implemented for down converter is not commonly used despite its excellent filtering performance. Similar to that of the dipole antenna, a fine-tuning of the filtering characteristics of a mechanical filter is often difficult to carry out with high precision as part of the manufacture processes. Like the dipole antenna packaged with plastic molding, a mechanical filter implemented for a down converter is also considered as expansive and inconvenient due to these difficulties.
Other than this high quality mechanical filter configuration, a down converter for semi-parabolic shaped antenna can also be manufactured on a printed circuit board (PCB), e.g., a FR4 PC board. One example of such a structure for build a down converter is disclosed in a U.S. Pat. No. 5,523,768, entitled "Integrated Feed and Down Converter Apparatus" by Hemmie et al. (issued on Jun. 4, 1996). An integrated semi-parabolic antenna/down converter multi-channel multi-point distribution system (MMDS) receiver is disclosed by Hemmie et al. which includes a support boom of a semi-parabolic antenna to contain the down converter electronics. Located at the focal area of the semi-parabolic antenna are a pair of driven feed elements which are directly connected to the printed circuit board carrying the down converter electronics. The down converter is formed in an elongated shape to fit entirely within the formed hollow interior of the support boom. The down converter comprises a first printed circuit board, which contains a RF filter located at the input end of the printed circuit board. The input to the RF filter circuit is directly connected to the pair of driven feed elements by soldering the legs of the driven feed elements directly to the input of the RF filter stage on the first printed circuit board. The RF filter is surrounded by an input ground shield, which covers the RF filter circuit. The shield is soldered to the top and bottom ground planes of the printed circuit board. At the opposite end of the printed circuit board is an output amplifier whose output is connected to a coax output lead. A coax ground shield engages the opposite end of the first printed circuit board in a perpendicular orientation so as to position the opposite and of the printed circuit board with the hollow interior.
While the printed circuit board (PCB) filters can be manufactured with simplified and automated procedures. The PCB filters provide the benefit of low cost implementation in the down converter. However, the PCB filters suffer from the disadvantages that energy transmission through the filters are impeded due to high dissipation over the PCB where large percents of signal energy are stored instead of transmitted through. The performance of signal filtering is also affected by temperature variations due to the fact that signal energy dissipation depends on the environmental temperature around the PCB. For these reasons, a PCB filter is not suitable for generating signals to be further processed by a low noise amplifier.
A different type of filter is manufactured by forming the filter on a ceramic substrate. Such a filter also suffers the same disadvantages as a PCB filter due to the fact that significant signal energy dissipation also incurs in the ceramic substrate. Again, the ceramic type of filters is not suitable for generating signals to be amplified by a low noise amplifier.
Therefore, a need still exists in the art of down converter for television signal transmission to provide a new structure and manufacture method to produce a new type down converter such that high quality low cost down conversion of the television signal can be achieved. It is desirable that a novel structure of a signal filter can be employed to provide the performance level of a mechanical filter in a down converter without requiring time consuming design and development efforts such that the manufacture cost of the down converter can be reduced. It is further desirable that such a down converter can provide high structural integrity suitable for reliable long-term outdoor operation.